Analysis Of Control Strategy For Offshore Wind Power Combined With Hybrid HVDC Network

Offshore wind power has the characteristics of rich wind energy resources,high development and utilization hours,stable wind speed,no occupation of land,small visual interference and suitable for large-scale development.Under the great initiative of developing renewable energy in the world,the offshore wind power has a broad development prospect.At the same time,the doubly fed induction generator has low production cost,mature technology and high reliability,and is suitable for offshore wind farm.Hybrid HVDC combines the advantages of VSC-HVDC and LCC-HVDC,and suitable for interconnection of offshore wind farms.Therefore,the analysis of the control strategy of offshore wind power through Hybrid HVDC connection has certain engineering significance.In order to suppress the failure of the commutation phase and improve transient stability when the inverter side LCC is connected to the weak AC network,this thesis proposes an improved control strategy based on the basic control strategy.The rectifier-side VSC converter adds constant DC voltage control to restrain the fluctuation of DC voltage by changing phase angle and improve the stability of the system.The inverter-side LCC converter adds constant turn-off angle control to prevent further reduction of the fault time gamma,so as to restrain commutation failure and accelerate the system to restore to steady-state operation.The purpose of suppressing the commutation failure and accelerating the recovery of the system to steady state operation is achieved.Therefore,the following aspects are studied in this thesis:Firstly,the mathematical model of the doubly fed induction generator(DFIG)is analyzed and the control strategy of the rotor side and the network side converter is designed,and the basic principle of the wind speed model,the maximum wind energy tracking control,the operation characteristics of the wind turbine and the control system of the pitch angle are analyzed in detail.Secondly,the operation principle,mathematical model,control strategy,basic structure and technical characteristics of VSC-HVDC and LCC-HVDC are discussed in detail,and the topology structure suitable for offshore wind power grid connection is analyzed.Based on PSCAD/EMTDC,the model of offshore wind power via VSC-LCC H-HVDC system is constructed,and corresponding basic control strategy and improved control strategy are designed.Finally,based on the PSCAD/EMTDC platform,the wind power flow changes under the wind speed change are analyzed,and transient characteristics of the system under AC bus faults at inverter side using basic control strategy and improved control strategy are simulated and analyzed.The effectiveness of the improved control strategy is verified.The results show that the active power and reactive power of wind farm output is changed with the fluctuation of wind speed,and the active power and reactive power can be quickly restored when the wind speed is stable.When AC bus fault occurs on the inverter side,the grid-connected system adopts improved control strategy,which can improve the stability of DC voltage,improve power quality,restrain the fluctuation of active power,reduce the impact of fault,and speed up the normal operation of the system.